Promo box: consider to use the user language

[afercia]

See Yoast/wordpress-seo#6186

Set user language in Spanish
Set site language in Dutch

Result:

The module is currently using get_locale(), maybe should use get_user_locale() ? Both?

Also, maybe the strings should be refactored a bit? While it makes sense to refer to the language used in the admin, there are now two languages so maybe the messages should clarify this?

[afercia]

One additional issue is that the locale name is... in English :)

[afercia]

About the GlotPress API, see future plans here: GlotPress/GlotPress#338